1. Field of the Invention
The present invention relates to a negative-electrode active material which has a low potential relative to lithium.
2. Description of Related Art
In recent years, a high-output and high-capacity power source which is applicable to low-emission vehicles, such as electrical vehicles and hybrid vehicles, is demanded from an environmental conservation perspective. In addition, in fields other than automobiles, the global diffusion of mobile tools, such as information-related devices and communication devices, stimulates the necessity of a power source which cart enhance the performance of the mobile tools. One of promising high-performance power sources is a lithium batter, which has a high energy density and is operable at a high voltage.
A lithium battery has a voltage equal to the difference between the potential of its positive-electrode material relative to lithium and the potential of its negative-electrode material relative to lithium. In other words, a high voltage and a high output can be obtained from the battery and the energy density increases when the potential of the negative-electrode material (in particular, the negative-electrode active material) is decreased. However, too much reduction in the potential of the negative-electrode active material relative to that of lithium may lead to precipitation of metal lithium in the batter. For example, graphite, which has a potential of 0 V relative to lithium, causes precipitation of metal lithium while in use.
One possible solution for preventing precipitation of metal lithium is to increase the operating voltage to a level at which metal lithium does not precipitate (in other words, to use a negative-electrode active material which has a high potential relative to lithium). Some batteries use spinel-type lithium titanate (Li4Ti5O12) or a partial substitution product thereof, which are typical examples of this type of negative-electrode active material (Japanese Patent Application Publication No. 2009-199798 (JP 2009-199798 A), Japanese Patent Application Publication No. 2011-086464 (JP 2011-086464 A), and Japanese Patent Application Publication No. 10-251020 (JP 10-251020 A)). Such materials have already been in practical use in some applications.
Spinel-type lithium titanate has a potential of 1.5 V relative to lithium and does not cause precipitation of metal lithium, but has too high a potential to be used as a negative-electrode active material. In other words, as battery which uses spinel-type lithium titanate as a negative-electrode active material is not suitable to obtain a high voltage and a high output.
TiO2 and Fe2O3 are some other examples, in addition to spinel-type lithium titanate, of transition metal oxides that can be used as a negative-electrode active material, but they have a high potential of approximately 1.8 V and 1.3 V, respectively, relative to lithium. In other words, a battery which uses TiO2 or Fe2O3 as a negative-electrode active material is not suitable either to obtain a high voltage and a high output.